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In a previous message, John wrote:

>Nominal discharge rate is usually the capacity divided by 20 (C/20)
>i.e. the cell is discharged in 20 hours.

>Discharging batteries in less than two hours will give substantially less
>apparent capacity. Someone suggested to me that a typical gelled cell
>lead-acid will only provide about 65% of the rated capacity when
>discharged in less than two hours. The non-gelled lead-cadmiums might
>be a bit better. I have not measured D cells of the NiCads persuation
>at 4A but I would guess a 4.4Ah cell which provides about 4.0 at 2Amps
>could maybe give 3.5Ah at 4 Amps for an overall drop of 20%. If the NiCads
>used by Ikelight were quick charging F cells, 4.5Ah could be the
>measured capacity.

Nominal capacity and how it's specified may depend on the battery
manufacturer.  Gates (now Hawker) specs its nicads at the 1C discharge
rate.  That is, if your battery is a 1.5 A.Hr battery, you can get 1.5
amps out of it for one hour.  Hawker claims that discharge capacity
is a function of three things: charge temperature, discharge temperature,
and discharge rate (discharge current).

Their curves for charge temperature show that one can expect rated
capacity for temperatures below about 30 degrees C, about 90 percent
at 50 degrees, and 80 percent at 60 degrees.  For discharge temperature,
look for about 50 percent capacity at -30 degrees, increasing roughly
linearly to 100 percent capacity at about 20 degrees.

As I said earlier, Hawker gives you full capacity from your battery
at the 1C rate.  At 2C, expect 90-97 percent; 3C, 85-95 percent.

So cold temperatures are bad, except when they're good, and discharging
at high rates is bad, but you have to discharge at *really* high rates
to see a lot of difference.  (For HAWKER batteries... your mileage,
of course, may vary.)

Tim

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